Spending two or three hours outdoors each day appears to lower a child’s risk of developing nearsightedness, although researchers can’t explain why.

A study published in this month’s issue of the journal Optometry and Vision Science reviewed several large studies examining nearsightedness, or myopia, in large populations. The studies agreed that the risk of myopia drops in children who spend more time outdoors. A child’s chances of becoming myopic if he or she has two myopic biological parents are about 6 in 10 for children who spend zero to five hours a week in outdoor activity. But the risk drops to two in 10 when outdoor activity exceeds 14 hours a week.